1. Field of the Invention
The present invention relates to a solid state image pickup device having a photoelectric conversion region for generating carrier by photoelectric conversion and accumulating the carrier, a method of manufacturing the solid state image pickup device, and a camera.
2. Related Background Art
In recent years, a MOS type sensor is used as a solid state image pickup device. The MOS type sensor has advantages over a CCD in that the MOS type sensor consumes lower power, requires lower drive power, and attains higher speed. It is, therefore, expected that such the MOS type sensor will grow in demand.
It is proposed to use such a MOS type sensor to expand a dynamic range of a solid state image pickup device (see Japanese Patent Application Laid-open No. 2001-186414 and U.S. Pat. No. 6,307,195 corresponding thereto). The MOS type sensor disclosed in the documents has a structure in which a plurality of pixels are arranged in a matrix. Each of the pixels has a photodiode, a floating diffusion (hereinafter also referred to FD when necessary) region, a transfer transistor for transferring carrier from the photodiode to the FD region, and a reset transistor for resetting the FD region to a predetermined potential.
In the MOS type sensor, a signal based on carrier accumulated in the photodiode is read first, and a signal based on carrier which flows out from the photodiode to be accumulated in the FD region is read. Then, the read signal is outputted through an analog amplifier.
There is also proposed another method. (See, Shigetoshi Sugawa, and 5 others, “A 100 db Dynamic Range CMOS Image Sensor Using a Lateral Overflow Integration Capacitor”, ISSCC 2005/SESSION19/IMAGES/19.4, DIGEST OF TECHNICAL PAPERS, 2005 IEEE International Solid-State Circuit Conference, Feb. 8, 2005, P352–353, 603).
In the CMOS area sensor shown in the above reference, an additional capacitor having a capacitance larger than that of the FD is further formed in each pixel. One terminal of the additional capacitor is connected to the FD through a switch, and the other terminal of the additional capacitor is connected to a ground. With this structure, when strong light expels carrier to flow out from the photodiode, the additional capacitor holds the carrier which has flowed out, making it possible to output a signal according to the amount of the carrier which flows out to thereby expand the dynamic range.
However, according to the conventional art described above, when very bright (strong) incident light comes in during the photodiode generates carrier to accumulate, a large amount of carrier flows out of the photodiode, and the FD region is not sufficient to accumulate all the carrier which flows out which leads to a problem in that the dynamic range can not be expanded.
Also, it may be possible to increase the surface area of the FD region to allow, the carrier which flows out from the photodiode to be accumulated thereon. However, increasing the area is contradictory to recent demand for more integration of a solid state image pickup device, which makes it difficult to increase the area.
Also, in providing the additional capacitor, a suitable structure thereof still needs further evaluation.